In U.S. Pat. No. 3,209,408 there is described a check valve for the plasticizer head of a plastic extruding machine, the check valve being formed at the end of a plastifying worm.
In this device, a worm is rotatable in the usual plastifying cylinder and homogenizes, liquifies or more generally plastifies a thermoplastic material while displacing this material to the end of the plastifying worm of a compartment at this end from which the material may be discharged, e.g. through an extrusion die or otherwise.
In this device, a conical tip is provided at the discharge end of the worm and flares rearwardly to a rearwardly facing shoulder adjoining a reduced cross section neck of this tip and a multiplicity of balls are arrayed around this neck against the shoulder. A valve ring is provided between these balls and another shoulder of the worm turned forwardly, the ring being received with lost motion between balls and this latter shoulder and a passage being defined between the ring and the neck through which the synthetic resin material can pass. The second shoulder is formed by a pressure ring against which the lost motion ring can bear and, to this end the two rings have complementary frustoconical surfaces.
The balls act as spacers holding the lost motion ring away from the forward shoulder, thereby permitting the flowable synthetic resin to pass through the spacers into the compartment ahead of the worm. The spacers of this reference thus are constituted by a peripheral array of balls similar in configuration or construction to and balls of a ball bearing so that the friction characteristics at the spacer are improved, i.e. the spacer does not create substantial friction so that the rotating worm need not entrain the lost motion or valving ring in rotation. The balls also serve to improve the kneading effect upon the synthetic resin as it flows past, between and around these balls.
In spite of precautions against wear afforded by the use of balls with the spacers, it has been found that this system has disadvantages because the relatively moving parts are subjected to significant forces which result in substantial wear. This requires replacement of the parts forming the check valve. It is also desirable to be able to easily replace the parts with different materials when different rheological properties are processed. For example, a replacement of the parts of the valve may be required to reduce the flow cross section or increase the latter.
In the earlier check valve arrangement, the replacement of the valve parts, namely the balls and the lost motion or valving member involve a complex operation in which the conical tip of the worm had to be removed, screw connections had to be detached from one another, etc. and the disassembly operations required special tools. In German patent document DE-AS No. 19 24 040, there is described a check valve located at the end of a plastifier worm whereby the replacement of the valving member or sleeve is greatly simplified. In this arrangement, an eccentric formation of a conical tip and an appropriate dimension of the clearance between the sleeve and the neck upstream of this tip, enables the sleeve to be radially shifted and withdrawn over the conical tip.
This system for dismounting the valving sleeve or ring has a considerable advantage in that no tools are required, but the eccentric arrangement presents a problem in that the flow distribution is nonuniform, canting of the ring can occur and even the wear of the device is not uniform. Another obvious disadvantage is the high cost of fabricating the device with the eccentric tip.